"SWEET as sugar" and "sour as vinegar" are among the most common comparisons in our language, and the two substances chosen to represent these opposite qualities are popularly deemed as unlike as they can well be. Yet it is one of the marvels of chemistry that the sourest substance with which we are familiar is made from the sweetest. By the action of a ferment, the sugar in some sweet liquid is turned first to alcohol, and the alcohol then changes to acetic acid, which is the acid in vinegar.

In Great Britain, vinegar, until recently, has been manufactured almost entirely from malt—a wort, or sugary solution, weaker than is employed for beer, being first made. Of late years, glucose, cane-sugar, and molasses, have been largely used. British "proof-vinegar" contains 4.6 per cent of anhydrous acid, A notion formerly prevailed that sulphuric acid acted as a preservative to vinegar, and one tenth of one per cent was allowed to be added. Makers continued the practice after they knew that it had no such effect, as it increased the apparent strength of their vinegar at a slight cost. This addition is now an illegal adulteration.

In France, and elsewhere in Europe, the manufacturer starts with an alcoholic liquid, already partly acetified—light wines that have turned sour being generally employed. The French name, vinaigre, from which the English word vinegar is derived, means sour wine. Two sorts are produced—white-wine and red-wine vinegar—the former being generally preferred. These are fine-flavored and somewhat stronger than the malt-vinegar of Great Britain. Six and one half to seven per cent of acid have been found in French vinegars. Sour ale and beer do not yield good vinegar.

In the United States, cider vinegar has long held the preference, and, if the cider has been made from sound, sweet apples, the vinegar has a very agreeable flavor and color. The old-fashioned way which is followed by farmers in making vinegar is to set out-of-doors in the spring a barrel of cider which has become too "hard" and sour to drink, from the sugar partly turning to alcohol and acetic acid. The bung is taken out of the barrel, and the bung-hole is loosely stopped by sticking the neck of a large bottle into it. Such exposure to the air at a warm temperature effects the conversion of the cider to vinegar in three or four months. The change goes on very slowly, because the air can act only on the surface of the liquid, and fresh portions of alcohol are brought to the surface only as the newly formed acid sinks and mingles with the liquid below. The best cider-vinegar is made from new cider, and it is well to cause several fermentations to take place by adding a fresh quantity of cider every two weeks.

Vinegar is chemically a dilute solution of acetic acid, containing minute quantities of fragrant ethers, which give it its odor, and some brownish substance, to which is due its color. Other matters, derived from the liquid from which the vinegar is made, are sometimes accidentally present, as sugar, gum, starch, cream of tartar, and other salts, etc. It usually consists of between ninety-three and ninety-seven per cent of water, the rest being acid, except a fraction of a per cent of solids. The transformation of the sugar in fruit juices or sirups to acetic acid takes place according to the following chemical reactions:

Sugar.

Alcohol.

Carbonic dioxide.

C6H12O6

=

2C2H6O

+

2CO2

The bubbles which appear when cider is "working" are carbonic dioxide. In the conversion of alcohol to acetic acid, a subtance called aldehyde is first formed. The oxygen for these changes is taken from the air:

Alcohol.

Oxygen.

Aldehyde.

Water.

C2H6O

+

O

=

C2H4O

+

H2O

Acetic acid.

C2H4O

+

O

=

C2H4O2

By substantially the same slow process as that still employed in the household was all vinegar obtained from the time of Moses, or earlier, down to 1822. In 1814 Berzelius had found out the chemical composition of acetic acid, and De Saussure that of alcohol; so that, after Doebereiner had discovered that a weak solution of alcohol exposed to the air in contact with platinum-black was converted to acetic acid, he was enabled to set forth the theory on which depends the modern "quick process" of vinegar-making—the method now regularly employed in the vinegar-factories of Europe and America. The essential feature of this process consists in bringing the alcoholic solution into intimate contact with the air by causing it to trickle through a mass of loose material, which effects the acetification in from twenty-four to forty-eight hours. The operation is carried on in wooden tubs, six to ten or more feet high, called generators. Around the sides of the generator, a few inches above the bottom, is a ring of air-holes. Just above the air-holes is a perforated false bottom, and from this nearly to the top the generator is filled with beech-wood shavings, which are closely curled so that they will not crush and prevent the air circulating freely through them. A few inches above the shavings is a wooden head or sieve, perforated with small holes, which serves to distribute the alcoholic liquid, or "wash," evenly over the shavings. Several air-pipes are inserted in the sieve, extending a few inches above and below it. The generator has a cover with a hole in the middle through which the wash is poured in, and the ascending current of air passes out. The vinegar-room is kept at a temperature between 70° and 90° Fahr. A high temperature and large supply of air hasten the operation, but cause loss by the evaporation of the alcohol. If the temperature falls much below 60°, the acetification stops, and putrefaction sets in; while if too little fresh air is supplied, aldehyde, the half-way product mentioned above, instead of being promptly converted to acetic acid, is evaporated and lost. The presence of aldehyde in the air may be detected by its penetrating aroma and by the eyes smarting. The wash must be passed several times through the shavings, in order to effect its complete acetification.

Two kinds of vinegar are sold by grocers in the United States for domestic use—cider-vinegar and white-wine vinegar. Both kinds are made in factories by the process just described. Massachusetts, New York, and some other Eastern States have laws concerning vinegar. In these States the cider-vinegar may be depended on as being really made from cider, for the risk of heavy penalties is incurred by offering anything else under this name. The laws require, also, that all vinegar shall contain four and a half per cent of acetic acid. Cider-vinegar contains a little malic acid, and will give a precipitate with acetate of lead. The absence of the precipitate shows that the sample is not cider-vinegar; but other vinegar, to which malic acid has been added, will, of coiirse, yield the precipitate. Many persons still retain a strong preference for cider-vinegar, but this, like the old preference for feather-beds, is gradually passing away. As rotten apples, and more or less of other kinds of dirt, commonly go into the cider-mill with the sound fruit, and no thorough purification of the product is attempted, not much can be said for home-made cider-vinegar on the score of purity. Many times as much white-wine as cider vinegar is now consumed in the United States, The white-wine vinegar, however, is not made from white wine, as that beverage is not sufficiently abundant in this country to supply the demand of the vinegar-manufacture. Until recently, manufacturers started with whisky, rum, or other alcoholic liquor, but they are now allowed to produce their own spirits. In the East, molasses, and in the West a wort from grain, is first fermented in a vinegar-still—an apparatus having no worm—and a liquor containing fifteen to twenty per cent of alcohol is produced. The liquor is then converted to vinegar in the usual way. This vinegar is perfectly colorless, and the brownish color which the consumer expects in vinegar is given to it by the addition of some harmless substance, as burned sugar, or an infusion of roasted barley-malt. Cider-vinegar has an agreeable flavor, due to the presence of acetic ether and malic acid. Vinegar from well-flavored wines is the most agreeable, as the ethers which give the bouquet to the wine produce a pleasant flavor in the resulting vinegar. Whisky containing fusel-oil yields a pleasant vinegar, as the fusel-oil during the acetification is decomposed into fragrant ethers. Vinegar is flavored artificially by adding to the last wash oil of cloves, or some fragrant ether.

A recipe is given in Ure's "Dictionary," by which it is said that an excellent vinegar for domestic use can be made. To each gallon of a sirup, containing one and a quarter pound of sugar to a gallon of water, is added one quarter of a pint of good yeast. The liquid is kept at a temperature of from 75° to 80° Fahr. for two or three days, and is then racked off from the sediment into the ripening-cask, where one ounce of cream of tartar and one ounce of crushed raisins for each gallon is mixed in. When the vinegar is freed from any sweet taste, it is drawn off clear into bottles and closely corked.

Vinegar should not be kept in metallic vessels except those of silver or perfectly clean copper. Earthenware glazed with oxide of lead (litharge) should never be used, but salt-glazed ware is safe. Vinegar is rarely adulterated with sulphuric acid; and oxalic acid, which is a violent poison, has also been found in it. According to the "United States Dispensatory," if vinegar is evaporated in contact with white sugar, or on white paper, the presence of free sulphuric acid will be indicated by charring. Such acrid substances as red pepper and mustard are sometimes added to vinegar to increase its apparent sharpness. They may be detected by their biting taste after evaporating a portion of the vinegar to a small bulk. Consumers need have little fear of adulterations, however, if their vinegar comes from ordinarily reputable dealers; besides, genuine vinegar can be made more cheaply than any passable imitation. There is more chance of unwholesome vinegar coming into the household in pickles and catchups than when the vinegar is bought alone.

By distillation vinegar is deprived of its coloring and other non-volatile matters. The product is always weaker than the vinegar from which it is derived, as the boiling-point of strong acetic acid is above that of water, and it contains small quantities of alcohol and empyreumatic bodies formed during the operation. Distilled vinegar was formerly used in pharmacy, but dilute acetic acid has now taken its place.

The acetic acid used in the arts is not obtained from the acetification of alcoholic liquors, but from the destructive distillation of wood, generally in the form of sawdust. It is called commercially pyroligne, ous acid, or wood-vinegar, and contains as impurities tar, wood-spirit-etc, which give it an empyreumatic or smoky odor, and which make it superior to other vinegar for preserving meats, pickles, etc. It is purified, and with the addition of coloring and flavoring matters has been sold for culinary use. As the complete purification is an expensive process, there is danger that this vinegar, if sold at a low price, will contain unwholesome substances.

The value of vinegar as a condiment depends on the fact that acetic acid dissolves gelatin, fibrin, and albumen; hence it aids in digesting young meats, fish, lobsters, and hard-boiled eggs. The acid assists also in the conversion of cellulose into sugar, which is the first stage in the digestion of the green leaves used in salads. It is a mistake to use vinegar on beans, for it renders insoluble the legumin, which is their chief nutritive constituent. Oil, pepper, mustard, and a little white wine make the best dressing for beans. It has been proved that some vegetable acid is necessary for the preservation of health, as long continuance in a diet lacking such acids produces scurvy. Vinegar will partly supply this lack, but not wholly, for it will not prevent or cure scurvy, A craving for acid is better satisfied by fruit or acid vegetables. Those young girls who indulge largely in such indigestible articles as pickled limes, cucumbers, etc., would enjoy better health if they should eat instead sour apples, tomatoes, and rhubarb- and cranberry-sauce. The habitual use of vinegar in considerable quantities leads to dyspepsia; the form becomes wasted, on account of insufficient nutrition; and death has been known to result.

Vinegar is used in medicine for its astringent action, being employed locally to check haemorrhage. It is also a refrigerant, for sponging the skin with diluted vinegar has a cooling effect. The heat and pain of sprains and bruises are relieved by applying to the place brown paper soaked in diluted vinegar. This use of vinegar is celebrated in the lines of a certain well-remembered lyric:

⁠"And Jill had the job⁠To plaster his nob
With vinegar and brown paper."

Aromatic vinegar, called also "Vinegar of the Four Thieves," Marseilles vinegar, or camphorated acetic acid, is strong acetic acid, in which are dissolved certain essential oils, and sometimes camphor. It is said to have been used by a band of four thieves, during a plague at Marseilles, to protect them from infection while plundering the houses and bodies of the dead. It is now used only in smelling-bottles, or vinaigrettes, for cases of fainting, a bit of sponge or some crystals of sulphate of potassium being put into the bottle and moistened with the liquid. Aromatic vinegar is very fragrant and volatile, and must be kept in closely stoppered bottles. A variety of recipes for it are given: that especially recommended in the "United States Dispensatory" is one and a half fluid drachm best oil of rose-geranium, fifteen minims oil of cloves, and four fluid ounces glacial acetic acid.

The tough, leathery substance, commonly called "mother," which forms in vinegar, is one of the many fungi whose spores float in the air, settle as dust on exposed objects, and fall into exposed liquids, ready to grow into a bulky plant when conditions favor. The exact position of the vinegar-plant among the fungi has not been settled. Turpin, Berkeley, and others, say that it is the abnormally developed mycelium, or vegetative part, of Penicillium glaucum, of which common mold is the reproductive part. Pasteur and others maintain that it is a distinct species, calling it by the name Mycoderma aceti, and state that common mold frequently grows on its surface. Under the microscope it has been found to exhibit two forms—the minute, rounded particles called micrococci, and the rod-like forms known as bacilli. The vinegar-plant develops during the process of acetification, and its presence tends to accelerate the operation. Manufacturers get rid of it as soon as possible, for it interferes with the flow of the vinegar through their apparatus. It grows on the surface of the vinegar, and if not disturbed will cover the whole surface, conforming to the shape of the vessel. It has been known to reach a thickness of half an inch. The mycoderm seems to have an oxidizing action, and so, when the alcohol in the liquid fails, it probably grows at the expense of the acetic acid, converting it to carbonic dioxide and water. There is a popular notion that the presence of "mother" shows that the vinegar is made from cider, and is of good quality, but the vinegar-plant appears also in vinegar made from molasses, and it is really as undesirable in vinegar as mold on bread.

The little, wriggling creatures which swarm in some vinegars have been credited by uneducated persons with being the "life" of the vinegar. In one sense they are, but their presence is in no way beneficial. These vinegar-eels (Anguillula accti), as they are called, are developed in most fruits, and hence readily find their way into vinegar made from fruit-juices. Vinegar which contains them must contain also as impurity some mucilaginous or albuminous matter, or the eels would have no food and could not exist. They need air also, and they have been observed engaged in a curious struggle with the mycoderm on the surface. The plant tends to prevent their obtaining the requisite supply of air, and the eels were seen combining their efforts to submerge it. They may be killed by heating the vinegar to 128° Fahr., or by adding boracic acid. Vinegar when long kept, especially if exposed to the air, putrefies and becomes ropy, losing its acidity, and acquiring an unpleasant smell; the presence of the vinegar-plant, vinegar-eels, or other foreign substances, is liable to induce putrefaction, especially if the vinegar is weak.